The present invention generally relates to the production of small or so-called “nano” fibers or droplets, which may be “spun” as fibers or “sprayed” as droplets by applying high electrostatic fields to liquid filled spraying tips, producing a Taylor cone at the tip opening. Thandavamoorthy Subbiath, G. S. Bhat, R. W Tock and S. S. Ramkumar, in the article, “Electrospinning of Nanofibers”, Journal of Applied Polymer Science, Vol. 96, 557-569 (2205), Wiley Periodicals, Inc., is instructive in this field. As the aforementioned article points out at page 561, there has been a debate on the potential and practicality of scaling up the technology to produce nanofibers at deposition rates required for commercial application.
Much of the reported basic R&D on the electrospinning of nanofibers has utilized a single spraying tube (typically a square cut tip end on a hollow hypodermic tube). In that prior art, the liquid flow into individual tips is typically regulated using a positive displacement pump (one pump per needle). If a positive displacement liquid tip flow is not provided individually to each spinning needle, the flow of liquid into the electrospinning orifices may be quite unstable. In order to reach commercial deposition rates, the inventor envisions the need for thousands of spraying orifices comprising an “Electrospinning Array”—the use of individual positive displacement pumps becomes impractical when this many tips are employed.
U.S. Pat. No. 6,713,001 teaches the use of separate positive displacement pumps, as well as altering the local electric fields of selected tips. Although the '001 proposes that a pressured liquid or a single positive displacement pump alone can be utilized to make spinning arrays, the only examples there utilize a single spraying tip fed by a positive displacement pump. It is the inventor's opinion that a single pressurized fluid or a single positive displacement pump cannot feed a practical large spinning array consisting of many individual tubes, which are otherwise unrestricted in their flow. This is opined because the flow rate of each individual unrestricted tip is inherently unstable vis-à-vis its neighbor tube. Changes in the electrostatic field on one tip caused by changes in the charged fibers or droplets in the gap (created partially by neighboring tip(s) spinning or spraying) affects that tip's flow by electrostatically affecting the effective surface tension balance at that tip's fluid projection. This in turn affects the flow (effective pressure) into other tips and, thus, the instability is maintained.
In an attempt to work around the flow instabilities alluded to above, Kim and Park (WO 2005/090653 A1) teach an array of tips spinning upward against gravity with each tip provided with excess liquid. The excess (dripping) flow, then, is individually collected in a scavenging gap, which is coaxial to each spinning tip. The excess liquid drips do not then contaminate the product onto which the spun fibers are being applied. Kim and Park also teach the use of air flow in yet another gap, yet coaxial to the spinning tip to keep the Taylor cone producing tip liquid lofted against gravity and thereby shaped to enable the startup of Taylor spinning. Kim and Park also teach the use of a funnel shaped tip to aid in shaping the Taylor pool. The collection of the excess flow from many tips, all elevated at high voltage with respect to the product, means that the collected fluid needs to pass through an insulating “liquid drop isolator” for return to the sourcing liquid pump. The teachings of Kim and Park, thereby, result in a complicated head, which contains many fluid flow paths, many flow adjustments, and precision machined parts to simply keep the drippings from reaching the product. This inventor notes that a drawing in WO 2005/090653 A1 shows the fluid path leading to the spraying tip, as a very thin line, and might be construed to be a capillary. No claims are made concerning this path and it would be most difficult to form (drill) a working capillary having appropriate length to diameter ratios.
Andrady, et al. in Patent Application Publication US 2005/0224998 A1 discloses an attempt to control fluid flows in a plurality of spinning (extrusion) tips through the use of a common electrode within the fluid source manifold.